Method of making an electrical resistor



9 I CORNELIUS Y. 1:: HUANG 3,390,452

ETHOD OF MAKING ANELECTRICAL RESISTOR Filed March 29, 1963 INVENTOR. CORNELIUS X 0. HUANG Elma/Jaw ATTORNEY United States Patent 3,390,452 METHOD OF MAKING AN ELECTRICAL RESISTOR Cornelius Y. D. Huang, Upper Darby, Pa., assignor to IRC, Inc., a corporation of Delaware Filed Mar. 29, 1963, Ser. No. 268,941 5 Claims. (Cl. 29-613) The present invention relates to an electrical resistor and methods of making the same. More particularly, the present invention relates to a deposited carbon resistor and methods of making the same.

A deposited carbon electrical resistor is a resistor in which the resistance material is carbon formed by pyrolytically decomposing a carbon containing gas, such as the hydrocarbon gases, to deposit carbon on a substrate of an electrical insulating material. Heretofore, the carbon resistance material was coated as a relatively thin layer on the surface of the substrate. To form such resistors of various resistance values, a groove was cut in the resistance material layer to form a relatively narrow, elongated resistance path. If the resistance material layer was coated on a flat surface a groove was cut in the resistance material layer to form a sinuous resistance path. If the resistance material layer was coated on the surface of a cylindrical substrate, a helical groove was cut in the resistance material layer as shown in US. Letters Patent No. 2,838,639 to G. V. Planer et al., dated June 10, 1958, entitled Film Resistor Spirallising.

Although such deposited carbon resistors generally have good operating characteristics so as to provide commercially acceptable resistors, it has been found that such resistors are subject to catastrophic failures. By catastrophic failure it is meant that the resistor becomes open circuited so that it is inoperative. Such failures result from the fact that the resistance material is in the form of a relatively thin and narrow path. Under certain extreme operating conditions the resistance material can oxidize to the extent that the resistance path becomes completely broken so as to open circuit the resistor.

It is an object of the present invention to provide a deposited carbon resistor which is free from catastrophic failures.

It is another object of the present invention to provide a deposited carbon resistor in which the resistance path is dispersed throughout the bulk of the body of a substrate to provide a multiplicity of interconnected resistance paths.

It is still another object of the present invention to provide a deposited carbon resistor comprising a porous ceramic substrate with the carbon resistance layer coated on the surface of the pores to provide a multiplicity of interconnected resistance paths.

It is a further object of the present invention to provide methods of making a deposited carbon resistor in which the resistance path is dispersed throughout the bulk of the body of a substrate.

It is a still further object of the present invention to provide methods of making a deposited carbon resistor comprising a porous ceramic substrate having a carbon resistance layer coated on the surface of the pores.

Other objects will appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps withrespect to each of the others, and the article possessing the features, properties, and the relation of elements, which are exemplified in the following detailed disclosure, and the scope of the invention will be indicatedin the claims.

The drawing is an elevational view, partially in section, of the resistor of the present invention.

Referring to the drawing, the resistor of the present invention is generally designated as 10. Resistor 10 com- 3,390,452 Patented July 2, 1968 prises a body 12 of a fixed ceramic material. Body 12 is provided with a plurality of interconnected pores 14 there- .throughout. A layer 16 of carbon is coated on the surface of the pores so as to provide a multiplicity of interconnected resistance paths extending from one end of the body 12 to the other end thereof. The ends of the body 12 are coated with termination layers 18 of an electrically conductive metal, such as silver. The termination layers 18 contact the carbon resistance layer 16 so as to be electrically connected thereto. Headed terminal wires 20 are secured to the termination layers 18.

The resistor of the present invention is made by mixing together particles of a ceramic material, a binder and a pore forming material. The materials can be mixed together in a ball mill to obtain a thorough mixture of the materials. The mixture is then formed into bodies of the desired shape, such as by extrusion or cold compression molding. The bodies are then heated in air to a temperature suflicient to decompose the pore forming material so as to provide the body with a myriad of interconnected pores. The porous body is then placed in a chamber which is flushed with an inert gas, such as nitrogen, to remove all air therefrom. A mixture of a hydrocarbon gas such as methane, and an inert gas, such as nitrogen, is continuously passed through the chamber which was previously heated to the decomposition temperature of the hydrocarbon gas. As the hydrocarbon gas passes through the heated chamber, it is decomposed to deposit carbon in the pores of the ceramic body. Thus, the surfaces of the pores in the body are coated with a layer of carbon to form a multiplicity of interconnected resistance paths extending through the body. By varying the rate of flow of the hydrocarbon gas and the deposition time, the amount of carbon deposited in the body will vary so as to vary the resistance value of the resistor. If the temperatures to which the body was heated during the pore forming step or the coating operation were not high enough to completely cure the ceramic body, the coated body can then be heated to a temperature sufiicient to complete the curing of the body. This should be carried out either in a carbon bed or an inert atmosphere to prevent oxidation of the carbon coating. The resistors can then be provided with a suitable termination and encapsulated in a protective casing or jacket.

The ceramic material of the resistor of the present invention may be any of the well known electrical insulating ceramics. Examples of some ceramics which have been found satisfactory are a mixture of aluminum oxide (A1 0 and a borosilicate glass, and an alkaline earth porcelain comprising silica (SiO barium carbonate (BaCO strontium carbonate (SrCO calcium carbonate (CaCO magnesium carbonate (MgCO and a clay (Al O -2SiO -2H O). Although any of the inorganic binders may be used, magnesium aluminum silicate has been found to be satisfactory. The pore forming material comprises either magnesium carbonate (MgCO or carbon. The amount of magnesium carbonate which may be used in the mixture is between 2% and 20%, by weight, and the carbon in the amount of 1% to 12% by weight. The amount of the binder in the mixture may be between 3% and 15% by weight. The remaining of the mixture is the ceramic.

If magnesium carbonate is used as the pore forming material, when the formed body is heated, the magnesium carbonate decomposes to magnesium oxide and carbon dioxide, and thereby forms the interconnected pores throughout the ceramic body. If the temperature in the coating chamber for decomposing the hydrocarbon coating gas is higher than the decomposition temperature of the magnesium oxide, the step of forming the pores in the ceramic body can be carried out in the coating Example I A batch of by weight 80% aluminum oxide (A1 8% borosilicate glass particles, 8% magnesium aluminum silicate and 4% magnesium carbonate (MgCO was mixed in a ball mill for 20 hours. The mixture was then completely dried, and then enough water was added to the mixture to make it sutficiently plastic for extrusion. The plastic material was then extruded into cylindrical rods which were cut to lengths of approximately 4 to 6 inches. The rods were then heated in air at a temperature of approximately 1000 C. for 30 minutes to decompose the magnesium carbonate and form the pores in the rods.

The porous rods were then divided into four groups. Each of the groups of rods were separately placed in a coating chamber from which the air was flushed by means of a flow of nitrogen. The chamber was then heated to a temperature of approximately 2200 F. and a mixture of methane and nitrogen was passed through the chamber. The flow rate of nitrogen was 5000 ml./min. The fiow rate of methane was varied for each of the groups of rods, and the specific flow rate of methane for each group is shown in Table I. The methane-nitrogen mixture was passed through the heated chamber and over the rods for a period of 1 /2 hours, after which time the rods were removed from the chamber. The rods were then cut into resistor bodies which were approximately 0.062 inch in diameter and 0.145 inch long. The resistor bodies formed from each group of rods had an average resistance 'value and temperature coefiicient of resistance as shown in Table I.

TABLE I Methane ohms (ml./min.) +25 C. to +25 0. to

Example II Resistors were made by the procedure of Example I except that the bodies were formed from a mixture of by weight 78% aluminum oxide (A1 0 8% borosilicate glass, 8% magnesium aluminum silicate and 6% magnesium carbonate (MgCO The methane flow rates used in the coating operation, and the average resistance values and temperature coetficient of resistance of the resistors of each group was as shown in Table II.

TABLE II Temperature Coetficient of Flow rate of Resistance, Resist. (percent) Methane ohms (ml./min.) +25 C. to +25 C. to

Example III Resistors were made by the procedure of Example I except that the bodies were formed from a mixture of by weight 92.15% alkaline earth porcelain, 6% magnesium aluminum silicate and 1.85% carbon. The alkaline earth porcelain comprised 15% silica (SiO 50% clay 4 (Al O .2SiO .2H O) and 35% of a mixture of alkaline earth carbonate and clay. The mixture of the alkaline earth carbonate and clay included 10% barium carbonate (BaCO 10% strontium carbonate (SrCO 10% calcium carbonate (CaCo 10% magnesium carbonate (MgCO and clay.

The extruded rods formed from this composition were heated in air at a temperature of 1100 C. for 2 hours to oxidize the carbon particles in the rods, and thereby form the porous ceramic rods. The porous rods were divided into five groups. Each group of rods was coated with carbon in the manner described in Example I. The flow rate of the methane used for coating each of the groups of rods is shown in Table III. The coated rods were embedded in a graphite bed and heated to a temperature of 1260" C. for 8 hours to completely cure the ceramic body without oxidizing the carbon coating. The rods were then cut into the resistor bodies. The average resistance value and temperature coefificient of resistance of the resistors formed from each group of rods is shown in Table III.

TAB LE III Temperature Coefficient of Flow Rate of Resistance, Resist. (percent) As can be seen from Examples I and II and their related tables, varying the content of the pore forming material in the body has little or no afiect on the resistance value of the completed resistor. Likewise, it was found that the temperature that the bodies are heated to decompose the pore forming material does not affect the final resistance value. However, as can be seen from all the examples, varying the flow of methane during the coating operation does affect the resistance value of the resistors with a decrease in the flow rate increasing the resistance value obtained. Although the content of the pore forming material does not affect the resistance value of the resistor, it does affect the strength of the ceramic body. The higher the content of the pore forming material the greater the numher and/ or size of the pores formed, and the weaker the body. It has been found that to provide a porous ceramic body of sufficient strength in producing electrical resistors, the maximum amount of magnesium carbonate which can be used to form the pores is approximately 20% and the maximum amount of carbon is approximately 12%.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. A method of making an electrical resistor comprising the steps of thoroughly mixing together particles of a ceramic material, a binder and a pore forming material selected from the group consisting of magnesium carbonate and carbon, forming said mixture into a compressed body, heating said body to a temperature sufiicient to decompose said pore forming material but below the curing temperature of the ceramic material to provide said body with interconnected pores extending throughout the body, exposing said body to an atmosphere of a hydrocarbon gas, heating said gas to a temperature at which said gas decomposes to deposit on the surface of the pores in said body a layer of carbon and then heating said coated body at the temperature at which the ceramic material is cured.

2. The method in accordance with claim 1 in which the coated body is finally heated in a bed of carbon to completely cure the ceramic material without oxidizing the carbon layer.

3. The method in accordance with claim 1 in which 1 the coated body is finally heated in an inert atmosphere to completely cure the ceramic material without oxidizing the carbon layer.

4. A method of making an electrical resistor comprising the steps of thoroughly mixing together 95% to 65% by weight particles of a ceramic material, 3% to 15% of a binder, and 2% to 20% of magnesium carbonate,

body to an atmosphere of a hydrocarbon gas, heating said gas to a temperature'at which said gas decomposes todcposit on the surface of the poresv in said body a layer of carbon, and then heating said coated body at the temperature at which the ceramic material is cured.

5. A method of makingan electrical resistor comprising the steps of thoroughly mixing together 95% to 73% by weight particles of a ceramic material, 3% to 15% of a binder, and 1% to 12% of carbon, forming said mixture into a compressed body, heating said body in air to a temperature sufilcient to oxidize the carbon but below the curing temperature 01' the ceramic material to UNITED STATES PATENTS 1,774,812 9/1930 Phelps 264-44 X I 1,842,186 1/1932 MeBerty.

1,978,323 10/1934 Power 29-155.7 X

2,057,431 10/1936 Hobrock 338308 X 2,200,521 5/1940 Siegel 338-308 X 2,487,581 11/1949 Palumbo 117-226 X 2,926,325 a 2/1960 Moore et al. 338-308 7 3,107,337 10/1963 Kohring '338-308 2,792,620 ,5/1957 Kohring 29--155.7

2,827,536 3/1958 Moore et al. 29-155.?

provide said body with interconnected pores extending there throughout, exposing said body to an atmosphere of a hydrocarbon gas, heating said gas to a temperature at which said gas decomposes to deposit on the surface of the pores of the body a layer of carbon, and then 1 heating said coated body at the temperature at which the ceramic material is cured.

7 References Cited 7 i CHARLIE T. MOON, Primary Emmi/tor, f

RICHARD M. WOOD, Examiner.

l. CLiNE, V. Y. MAYEWSKY, Assislum Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,390,452 July 2, 1968 Cornelius Y. D. Huang It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed' specification, line 5, "IRC, Inc. a corporation of Delaware" should read 'mw Inc. a corporation of signed' and sealed this 3rd day of March 1970.

(SEAL) Attcal:

Edward M. Fletcher, Jr.

Mlesling Officer Commissioner of Patent- 

1. A METHOD OF MAKING AN ELECTRICAL RESISTOR COMPRISING THE STEPS OF THOROUGHLY MIXING TOGETHER PARTICLES OF A CERAMIC MATERIAL, A BINDER AND A PORE FORMING MATERIAL SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM CARBONATE AND CARBON, FORMING SAID MIXTURE INTO A COMPRESSED BODY, HEATING SAID BODY TO A TEMPERATURE SUFFICIENT TO DECOMPOSE SAID PORE FORMING MATERIAL BUT BELOW THE CURING TEMPERATURE OF THE CERAMIC MATERIAL TO PROVIDE SAID BODY WITH INTERCONNECTED PORES EXTENDING THROUGHOUT THE BODY, EXPOSING SAID BODY TO AN ATMOSPHERE OF A HYDROCARBON GAS, HEATING SAID GAS TO A TEMPERATURE AT WHICH SAID GAS DECOMPOSES TO DEPOSIT ON THE SURFACE OF THE PORES IN SAID BODY A LAYER OF CARBON AND THEN HEATING SAID COATED BODY AT THE TEMPERATURE AT WHICH THE CERAMIC MATERIAL IS CURED. 